The paper deals with the problem of safety evaluation of cryptographic modules used within safety-related control system for applications with increasing safety integrity level. The requirements to cryptographic techniques in safety-related communication for railway application are describe. The mainly part is oriented to description of mathematical apparatus for an error probability of cryptography code with a safety code, used in an additional safety communication layer. The practical results are related with the quantitative evaluation of an average error probability of code word for Euroradio protocol recommended for communication in European Train Control System.